This invention relates to creation of images and, more particularly, to generation of images on raster displays. These images can consist of textual and/or graphical information. Specifically, the invention is directed to a method and apparatus for digitally processing image data, which increase the speed or rate of generation of corresponding images on a raster display.
One type of raster display is a cathode ray tube (CRT) on which images are displayed by a technique known as raster scanning. Raster scanning involves driving a deflection control circuit which directs an electron beam modulated by image information onto discrete areas of luminescent material on a display screen. The image information determines whether or not each discrete luminescent area is illuminated. Typically, raster scanning involves sweeping the electron beam from the upper left hand corner of the screen horizontally across the screen to the right to selectively illuminate a horizontal row of discrete luminescent areas and repeating the process for each row of the screen from top to bottom, selectively illuminating each discrete luminescent area in accordance with the corresponding image information which modulates the electron beam.
The electron beam can be modulated in various ways depending on the manner in which the CRT is being used. One example is a television in which image information is transmitted through the atmosphere and detected by a television receiver which decodes the received image information and modulates the electron beam to display images on a screen. The deflection control circuit sweeps the electron beam to generate images on a television screen as many as 60 times a second.
CRTs are also used as displays for other purposes. One such use is in computer terminals. Here, images are displayed by sweeping the electron beam in the same way as in a television. Unlike television, however, the image information is not generally transmitted through the atmosphere, but rather is input to the computer at a local or remote location and stored in a screen memory. A display control processor feeds the stored image data in the screen memory to the CRT for modulating the electron beam to generate an image corresponding to the stored image data.
Another use of a CRT is in electronic instrumentation, such as an oscilloscope, spectrum analyzer, or network analyzer. These instruments measure characteristics of received signals transmitted through the atmosphere or responses of electronic devices connected to them. Typically, the measured information is processed and stored in a screen memory, similar to the way in which image data is stored in the screen memory for display on computer terminals.
Unlike computer terminals in which data is entered and displayed at relatively low rates or speeds, instruments make measurements at significantly higher speeds. For example, data can be entered in computer terminals by a keyboard at typing speed, say at an average of 80 characters a minute, whereas sophisticated instruments make measurements at a rate of between 300 to 3,600 times a minute.
In most instruments with CRTs, standard off-the-shelf graphics system processors are used to update the display, due to their relatively low cost (compared to custom graphics system processors or dedicated graphics engines). The resultant update rate is typically two to five times a second during normal measurement operation. Examples of such instruments include the Hewlett-Packard Company HP 4195A Network/Spectrum Analyzer, HP 54110 Color Digitizing Oscilloscope, and HP 70000 Modular Measurement System, as well as the Wiltron Company 360, Wiltron 561, and Wiltron 6409 network analyzers.
Achieving a fast display update rate is very important in many instrument applications. If the display cannot be updated as fast as measurements are made, the data collection process must be slowed down, or else the user of the instrument will not see the data that has been collected. The measurement traces will be updated sluggishly, making the instrument less responsive to the user. In addition, if the display is not updated quickly, the instrument will not have a "real-time" feel; that is, the images will dance in steps to the final displayed values rather than appear to move smoothly and instantaneously to those final values as the display is updated with new measurement data that has been collected. A display update rate of at least 10 to 20 updates a second (10 to 20 Hz) is needed in order to achieve a "real-time" feel.
One disadvantage of raster displays used in instruments is that pixels on the display screen must be written by the graphics system processor into a region of screen memory. The process of writing image data corresponding to a single line into the screen memory can require the graphics system processor to access hundreds of screen memory locations, consuming a significant amount of time.
In fast instruments, which make measurements at 3,000 or so times a minute (as fast as 60 Hz), changes in the measured data are not faithfully displayed on the CRT quickly because of limitations of the graphics system processor which is not able to quickly fill video memory at a rate that can be accommodated by the 60 Hz maximum update capability of the deflection control circuit of conventional CRTs. It is desirable that the rate or speed with which image data can be processed can be better matched to the display update capability of the deflection control circuit of the raster display so that changes in images can be more quickly updated on the raster display and perceived by the user.